The heritability of total plasma testosterone' levels, determined from blood samples, was examined in 160 adolescent twin pairs and their parents. Subjects were tested as part of a larger study of cardiovascular risk factors, conducted in Amsterdam. Each subject provided a sample of blood which was assayed to measure testosterone concentrations. Correlations of testosterone in monozygotic twins were higher than in dizygotic twins. No resemblance was found between testosterone values in fathers and those in their children and a moderate correlation was seen between mothers and their daughters. The lack of resemblance between family members of opposite sex suggests that different genetic factors influence plasma testosterone concentrations in men and women. In adolescent men, approximately 60% of the variance in testosterone levels is heritable. The lack of father-son resemblance suggests that different genetic factors may be expressed in adolescence and adulthood. In women, 40% of the variance in testosterone levels is heritable, both in adolescence and in adulthood.